In general, this invention relates to video reproduction apparatus in which parallel channels of video information are recorded on multitrack magnetic tape as FM video signals. More particularly, this invention relates to a multitrack magnetic tape video reproduction apparatus wherein a plurality of demodulators which demodulate a plurality of channels of video information reproduced from magnetic tape are provided with auto calibration circuitry to effect constraint of the demodulated video signals within a range of fixed peak-to-peak, white-to-black reference levels.
Various techniques have been proposed for the motion analysis of an event such as to detect malfunctions in high speed machinery, to analyze movements of an athlete, to test safety equipment, to study the shattering of an object, etc. Multitrack magnetic tape reproduction devices have been especially useful in the motion analysis of such fast moving phenomena in slow motion. Such a motion analysis device is disclosed in commonly assigned U.S. Pat. No. 4,496,995, issued Jan. 29, 1985. As disclosed in this Patent, the motion analyzer entails the recording of a large number of images during an event at high tape speed and high frame rate and then playing back the images more slowly at a lower frame rate and lower tape speed to analyze the movement which has occurred in step by step progression. The motion analyzer includes a video imager (camera), a variable speed multitrack magnetic tape processor and a video display monitor. A scene to be analyzed is focused on the imager which is read out in block format so that a frame of video information is read out in a sequence of blocks of a plurality of channels of video information that correspond to rows of photosites in the imager. The parallel channels of video information in each block are simultaneously recorded on magnetic tape in longitudinal parallel tracks thereon as frequency modulated video information signals. Upon playback the plurality of FM video signals are played back simultaneously and converted into a sequential signal which is displayed on a video monitor.
Since a plurality of FM modulators and demodulators are used during the recording and reproducing of the parallel signals, it is desirable that each of the channels of reproduced video information matches to each other with respect to peak-to-peak, white-to-black video range so that there is uniformity of displayed signal intensity from line-to-line and from block-to-block. Although, ideally, the modulators and demodulators adhere to nominal specifications, some variation can be expected from the modulators and demodulators in each of the several channels. Thus, it has been the practice to provide manually adjustable trimmers in the modulator and demodulator circuits in order to effect calibration of the plurality of modulators and demodulators with respect to each other to produce uniformity in signal intensity of displayed images.
Moreover, although ideally magnetic tape should be moved past the reproducing head at a constant speed in order to accurately reproduce the video signals from the tape, in reality, severely degraded or totally incomprehensible displayed video information may result from variations in the speed of the tape, due to localized tape stretch, tape vibration, etc. This timing error (flutter) introduced into the video signal is undesirable.